Light-emitting elements that emit light by applying an electric field thereto, such as electroluminescent (EL) elements and light-emitting diodes, are self-luminous, and thus have high visibility and can be made thin. For these reasons, such elements are attracting attention as lighting devices such as backlights and as display devices such as flat panel displays. Above all, organic EL elements that use organic compounds as emitters can be driven at low voltages, can be easily made large in area, and can easily obtain the desired emission color by selecting appropriate dyes, and thus are being actively developed as next-generation displays.
For an EL element using an organic emitter, blue light emission is obtained by, for example, applying a voltage of 30 V to a vapor-deposited anthracene film with a thickness of 1 μm or less (Thin Solid Films, 94 (1982) 171). However, this element did not achieve sufficient luminance even when a high voltage was applied to the element, and thus required further improvement in luminescence efficiency.
On the other hand, Tang et. al. made an element in which a transparent electrode (anode), a hole transport material layer, an electron transport luminescent material layer, and a cathode using a low work function metal were stacked on top of each other, so as to improve luminescence efficiency. The element achieved a luminance of 1000 cd/m2 with an applied voltage of 10 V or less (Appl. Phys. Lett., 51 (1987) 913).
Furthermore, there have been reported an element having a three-layer structure in which a luminescent material layer was sandwiched between a hole transport material layer and an electron transport material layer (Jpn. J. Appl. Phys., 27 (1988) L269), and an element that obtained light emission from a dye doped in a light-emitting layer (J. Appl. Phys., 65 (1989) 3610).
A cross-sectional view of the general configuration of a prior-art organic EL element is shown in FIG. 26. In the drawing, reference numeral 71 denotes a transparent substrate made, for example, of glass, plastic, or the like, reference numeral 72 denotes a transparent anode made, for example, of indium tin oxide (ITO), reference numeral 73 denotes a hole transport material layer made, for example, of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD), reference numeral 74 denotes an electron transport luminescent material layer made, for example, of tris(8-quinolinolato)aluminum (Alq3), and reference numeral 75 denotes a cathode made, for example, of an AlLi alloy. Reference numeral 76 denotes a light-emitting layer. When a voltage is applied to the element in the direction shown in the drawing, holes are injected from the anode 72 into the hole transport material layer 73, and electrons are injected from the cathode 75 into the electron transport luminescent material layer 74. The holes injected from the anode 72 pass through the hole transport material layer 73, and then are injected into the electron transport luminescent material layer 74. The holes and electrons are recombined in the electron transport luminescent material layer 74, thereby exciting molecules of Alq3, from which light emission is obtained.
The transparent anode made of ITO is typically formed by sputtering, electron-beam evaporation, or the like. The hole transport material layer and the electron transport luminescent material layer, made of organic substances such as TPD and Alq3, and the cathode made of an AlLi alloy or the like, are typically formed by resistive heating evaporation.
An example of light-emitting elements other than the above-mentioned organic EL element is an inorganic EL element. A cross-sectional view of the general configuration of an inorganic EL element is shown in FIG. 27. In the drawing, reference numeral 81 denotes a transparent substrate made, for example, of glass, reference numeral 82 denotes a transparent electrode made, for example, of ITO, reference numeral 83 denotes a first insulating material layer made, for example, of Ta2O5, reference numeral 84 denotes a luminescent material layer made, for example, of Mn-doped ZnS, reference numeral 85 denotes a second insulating material layer made, for example, of Ta2O5, and reference numeral 86 denotes a rear electrode made, for example, of Al. Reference numeral 87 denotes a light-emitting layer. When an alternating electric field is applied to both electrodes of the element, electrons moved from the interfaces between the insulating material layers and the luminescent material layer are accelerated, and thereby collide with and excite Mn, which is the luminescent center. Upon returning to the ground state, light emission occurs.
One of the factors contributing to the limited luminescence efficiency of these light-emitting elements is the external extraction efficiency of light emitted from the light-emitting layer (external extraction efficiency). For example, as shown in FIG. 28, of light emitted from an electron transport luminescent material layer 94 by application of a voltage to an anode 92 and a cathode 95, such light as to have angles equal to or greater than the critical angle is totally reflected at an interface between a hole transport material layer 93 and the transparent electrode 92, at an interface between the transparent electrode 92 and a transparent substrate 91, or at an interface between the transparent substrate 91 and air (i.e., a light-extracting surface), and therefore cannot be extracted outside the element. The external extraction efficiency is expressed by: 1/(2n2), where n is the refractive index of the luminescent material layer (Adv. Mater. 6 (1994) p.491). In a typical organic EL element, the refractive index of the luminescent material layer is about 1.6 and the external extraction efficiency is about 20%. In an inorganic EL element, when ZnS with a refractive index of about 2.3 is used as the luminescent material layer, the external extraction efficiency is about 10%. Hence, even when the internal quantum efficiency (which is the conversion efficiency of injected electric charges into light) is 100%, because of the limitation by external extraction efficiency, the external quantum efficiency turns out to be on the order of 10% to 20%.
Various methods have been investigated to improve external extraction efficiency. For example, there have been suggested: (1) a method of forming light reflective films at the edges of the substrate (see Japanese Unexamined Patent Publication No. 61-195588 and the like); (2) a method that uses a substrate having light condensing characteristics of lens or the like (see Japanese Examined Patent Publication No. 2670572, Japanese Unexamined Patent Publication No. 4-192290, Patent No. 2773720, Japanese Unexamined Patent Publication No. 10-172756, Japanese Unexamined Patent Publication No. 10-223367, and the like); and (3) a method in which a light-emitting layer or substrate is formed in a mesa form (see Japanese Unexamined Patent Publication No. 4-306589, Opt. Lett. vol. 27, No. 6 (1997) p.396, and the like).
The above-mentioned method (1) is such that light reflective films are provided at the edges of the substrate, whereby light, which is propagated mainly through the substrate and then diffracted from the edges of the substrate, is condensed onto a light-extracting surface. However, in the case, for example, of a so-called dot matrix display in which very small light-emitting elements are arranged in a matrix, it is very difficult to form such reflective films on every light-emitting element which corresponds to a unit pixel.
In the case of the above-mentioned method (2) in which the light extraction side of a substrate has a lens configuration, even when light-emitting elements and lenses are disposed at a one-to-one ratio in a dot matrix display such as one described above, because light emitted from a very small single light-emitting element is isotropically emitted, most of the light, which has passed through the substrate with a certain thickness and reached the light extraction side, is incident on the lens on the adjacent pixel side. Accordingly, the effect on the improvement of the efficiency of the light-emitting element, which is the object, is little, and besides, the problem of image blur occurs. In order to overcome the problem, there has been suggested a method of positioning light-emitting regions and lenses as close as possible to each other by, for example, implanting lenses in the substrate (Japanese Unexamined Patent Publication No. 10-172756). This method inhibits such image blur as to be described above and the like, but has difficulty in manufacturing.
The above-mentioned method (3) has difficulty in processing a substrate.